A wellhead for use with subterranean wells includes an improved tubing hanger including an improved electric power cable pack-off port that permits positioning an electric submergible pump (xe2x80x9cESPxe2x80x9d) power cable through the port in the tubing hanger. The improved wellhead permits installation of packing and compression rings within the power cable port to create a vapor-tight pressure seal around the outer cable jacket. The seal may be rated at pressures of at least 750 psia.
A wellhead is commonly used for suspending production tubing and casing inside the well-bore of an oil or gas well. Typically, a tubing hanger including female threads may be attached to the uppermost joint of production tubing to support the production tubing string and provide a seal between the tubing, the casing annulus and the atmosphere external to the well. The tubing hanger may engage a substantially complimentary receptacle port in the upper portion of the wellhead body. In a naturally flowing gas well, the hanger may include a tubing port, having a substantially coaxial lower portion and upper portion, both of which may be threaded, wherein the lower portion of the port may engage the uppermost threads of the suspended production tubing string and the upper portion of the port may engage a surface production line, valve or other production conduit, allowing gas or well fluids to pass through the well-head and into a pipeline or vessel. The wellhead body may also have two side ports to permit venting of gas vapors from within the annulus between the production tubing and production casing strings to a pipeline or vessel.
Another type of gas well may produce commercial quantities of gas only when an undesirable buildup of water is pumped out of the well-bore so as to reduce back-pressure on the producing formation. Shallow geologic coal bearing formations may contain a substantial supply of methane gas under relatively low reservoir pressure. This gas may have been considered an undesirable by-product, when compared to the value of the coal. If the equipment costs to complete wells drilled into these formations can be kept relatively low, as compared to a high-pressure gas or oil well, then this xe2x80x9ccoal-bed methane gasxe2x80x9d may become a commercially viable natural resource. Unfortunately, water is also frequently present and the down-hole reservoir gas pressure may be so low that gas may be trapped in the formation due to the hydrostatic head of the water. In most coal-bed methane wells, this hydrostatic head may be relieved by pumping the water out of the well-bore by one of several types of artificial lift.
A popular method of pumping water from this type of gas well utilizes an electrical submersible pump and integral electric motor, commonly referred to collectively as an ESP, suspended near the bottom of the well-bore by the production tubing which may be hung from the tubing head or tubing hanger. The water may be pumped through the production port in the tubing hanger and gas may be produced under natural reservoir pressure, up the tubing-casing annulus and out the side ports of the wellhead body. This method of pumping may also require that an ESP power cable be connected between the electric motor of the down-hole ESP and an electrical control panel on the surface. Ideally, in terms of simplicity and cost, in a low-pressure application, a continuous power cable is installed between the control panel and the down-hole pump or ESP. The wellhead should also permit the cable to pass through the top of the wellhead and effect a vapor tight seal so as to prevent valuable gas from being vented to the atmosphere in order to prevent waste of natural resources and to prevent a fire or explosion hazard around the wellhead.
The prior art fails to disclose a reliable and economical method for allowing a continuous ESP power cable to be positioned between a control panel and an ESP. A cost-effective system is desired to create a mechanically effective pneumatic seal at the wellhead. FIGS. 1 and 2 illustrate common prior art wellhead assemblies. The FIG. 1 wellhead may be commonly used on low and high-pressure oil and gas wells equipped for ESP pumping. The wellhead installation illustrated in FIG. 2 may be used on relatively higher-pressure oil and gas wells. Due to their complexity and cost, these type of wellheads may not be desirable for economically marginal low pressure gas or oil wells. In addition, mechanically fabricating and installing all of the components as illustrated in FIG. 1 may be rather difficult. The sealing effectiveness may also be problematic, particularly if all of the eccentric ports or penetrations do not perfectly align with respect to one another.
The wellhead assembly illustrated in FIG. 1 may typically be used in applications for annulus surface pressure ratings of up to 1500 psia. The ESP power cable may pass through the tubing hanger component of the wellhead as a continuous cable from the control panel through the well-head to the ESP motor. A second port or penetration may typically be provided in the metal and rubber packing plates of the tubing hanger, parallel to the threaded port suspending the production tubing. In addition, one or two additional ports may be provided in the tubing hanger to permit passage of capillary tubes to permit injection of well treatment chemicals and/or monitoring of surface pressure in the well annulus. A known drawback to this design is that the metal plates may require machining with multiple, eccentric xe2x80x9cpenetrations,xe2x80x9d and the packing components must also be manufactured with corresponding penetrations. Each cable sealing penetration must be sized and positioned to fit the outerjacket of the ESP cable, and must additionally precisely align with respect to one another. These numerous parts with eccentric penetrations may be relatively expensive to manufacture, due to the necessity for substantially exact alignment of the various eccentric penetrations with respect to the adjacent parts. These wellhead configurations may also be typically over-designed from both a pressure rating and cost standpoint for coal-bed methane gas producing wells or other low pressure oil or gas wells.
The wellhead assembly illustrated in FIG. 2 may be typically used on oil or gas wells presenting relatively high pressure in the wellbore annulus between the casing and tubing. Typically these well head configurations may have a pressure rating in the 3000 to 5000 psia range. At such pressures, corrosive, toxic and/or explosive gas can penetrate the armor or insulation of the ESP power cable, from within the wellbore, and may migrate to the surface and into the electrical control box creating a serious safety hazard. A means of physically truncating the power cable while permitting the passage of electricity may be required in these applications. This may be accomplished with costly and relatively complex additional hardware added to the wellhead, such as a double-ended plug or receptacle, commonly referred to as a xe2x80x9cpenetratorxe2x80x9d or mandrel. The power feed-through penetrator may be positioned in the wellhead and may include upper and lower detachable power connectors and an insulating and sealing dielectric material to create a pressure barrier while allowing electricity to be conducted through the wellhead. These additional components may cost many times more than the wellhead body and tubing hanger, thus precluding their applicability for use with coal-bed methane wells, from a economic standpoint.
This invention provides a cost effective, improved reliability wellhead for effectively sealing between a tubing hanger and an electrical cable for powering a downhole ESP. This invention may be particularly applicable to low pressure and/or marginally economically wells where cost considerations are of relatively increased concern. A tubing hanger is provided which includes a tubing port for passing produced fluid therethrough, and a cable port for positioning the electrical power cable for the ESP therethrough. All sealing between the tubing hanger and the cable may be substantially performed within the cable port, as opposed to above the cable port. Thereby, smaller, less costly, more precisely sized and easier to manufacture and install cable sealing components may be utilized.
Laboratory testing of embodiments of this invention, such as illustrated in FIGS. 3, 4, 5, 6 and 7, has demonstrated a wellhead capable of effecting a pneumatic, vapor tight seal around an ESP power cable, at differential pressures across the seal of at least 750 psia for a 24 hour period. Such testing has been performed using nitrogen gas, which exhibited no leakage around the outer cable jacket, where the cable exits the top of the wellhead. Alternative embodiment versions of wellheads according to this invention may provide sealing capabilities of at least 1500 psig.
It is an object of the present invention to provide a wellhead for use with an ESP, in a relatively low pressure well. This invention provides a wellhead that may be used with wellbore pressures of at least 500 psig.
It is an additional object of this invention to provide a wellhead for sealing with an electrical cable for powering an ESP in the wellbore, wherein the power cable may extend from the motor to a power source external to the wellbore, such as in a control panel.
According to the present invention it is an additional object to provide a tubing hanger supported within a wellhead body on an upper end of a wellbore, wherein the tubing hanger includes at least a tubing port and a cable port therein. A tubing string connected on a lower end to the pump may be connected on an upper end to the tubing hanger in fluid communication with the tubing port. The flexible power cable may be positioned through the tubing hanger cable port. A cable seal may be provided within the cable port to seal between the power cable and the tubing hanger. A packing gland may be included to compress the cable seal.
It is an object of this invention to provide a method of sealing the interior of a wellhead providing a cable port in a tubing hanger supported within the wellhead, wherein a flexible ESP power cable is positioned within the cable port. The method may include positioning a cable seal within the cable port to seal between the power cable and the tubing hanger. A packing gland may be moved with respect to the tubing hanger to compress and activate the cable seal.
It is a feature of the present invention that upper and/or lower compression rings may be provided within the cable port to assist in compression of the cable seal.
It is also a feature of the present invention that the packing gland and the tubing hanger may threadably engage on another to facilitate turning the packing gland to compress the cable seal.
It is still another feature of the present invention that a plurality of bolts and corresponding bolt holes in the tubing hanger may be included to compress the cable seal as the bolts are tightened. Compressive forces may be transferred from the bolts to the packing gland by an upper portion of the packing gland and/or by a packing gland retainer engaged with each of the bolts and the packing gland.
It is a feature of the present invention that the tubing hanger and cable sealing components are relatively simple and cost effective to manufacture.
It is also a feature of this invention that the sealing capabilities of this invention are reliable and simple to install and maintain.
It is an additional feature of this invention that the methods and components of this invention may be retro-fitted in existing wellheads and ESP installations.
Another feature of this invention is that it may be adapted to virtually any known ESP cable configuration, including multiple conductor, armored, round and flat cables.
It is an advantage of this invention is that the packing elements and the packing gland are smaller that prior art packing elements and glands. Adjustments may be effected with less effort and with improved sealing effectiveness as compared to prior art cable seals.
It is also a feature of this invention that the packing elements seal across less cross-sectional area and against less, lateral sealing surface area than prior art wellhead packoff seals for ESP installations.
It is an additional feature of this invention that the cable seal may be compressed by a variety of gland configurations. For example, in one embodiment, a packing gland may be threadably engaged within a portion of the cable port. In another embodiment, a packing gland may be threadably engaged to a portion of the tubing hanger other than in the cable port.
It is still another feature of this invention that a wellhead retainer cap is not required to effect a pneumatic seal with the cable and the tubing hanger in the cable port.
An additional feature of this invention is that a wellhead penetrator is not required, and an electrical power cable need not be segmented or cut at or near the tubing hanger to pass electrical power through the cable port.
It is an advantage of this invention to provide a cost-effective wellhead for economically sensitive ESP completions.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to figures in the accompanying drawings.